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Housing - Renovation: U values with secondary glazing

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  1.  
    • CommentAuthorbjd
    • CommentTimeSep 21st 2009
     
    My new house has a number of 1920's sash windows with internal secondary glazing - sliding panels with brushstrip seals around the edges (Everest I think, less than 10 years old).

    Just had a thermal study done to try and highlight the areas that need attention - the report has used a U value for these windows of 2! Older 4/12/4 dg windows have been rated at 2.8, so I find it hard to believe that 2 is achievable with the secondary glazing (gap is around 100mm).

    Has anyone got any experience of the typical U values when such glazing is used? It rather drastically affects the numbers in this study. I did a heating demand calc on the house (such as you would do to size a boiler) and used 3, but don't really have a clue.

    Creative DIY should allow me to renovate the windows at a low cost to around 1.5, but if we are already getting around 2 then it would be a waste of time - apart from sorting out the masses of condensation we get on the outer panes all winter.

    Thanks for your help.....
    • CommentAuthorEv
    • CommentTimeSep 21st 2009
     
    I'd be wary of taking thermal studies as gospel.

    Windows: http://www.historic-scotland.gov.uk/thermal-windows.pdf
  3.  
    ... interesting results for shutters... but what about interstitial condensation...?

    J
  4.  
    I know one co was quoting 2.0, maybe even 1.8, but I would not expect that to happen often. I suspect a huge deal of the efficacy of a 2ndary G unit is in whether or not the SG window is well d/stripped, and the 2nd pane is not having to work as a (cooled) draught-stop.
    • CommentAuthorbjd
    • CommentTimeSep 21st 2009
     
    Ev, Thanks for the link - it certainly answers my question - I am being wary of the study, hence the Q!

    Sounds like the existing 2dry glazing is doing a reasonable job, and would do a lot better if we draftproofed the sashes properly.

    Condensation is a problem on the inside of the outer panes, which is one reason for wanting to improve things - I can't work out if this is likely to be moisture from outside coming into the gap and warming/ cooling to a condensate, or moisture from inside (i.e. if the 2dry isn't doing a brilliant job at draftproofing). I suppose if failed dg units mist up - you have no hope with a secondary system.....
    •  
      CommentAuthorrogerwhit
    • CommentTimeSep 21st 2009
     
    On U-values, can we assume that your sec glazing is single glass, not double?

    With ref to draughtstripping, you DO want to vent the original outer window to the outside air to prevent condensation being trapped in the void, and seal the inner (secondary) window to prevent room air entering that same void. A certain amount of draughtproofing to the outer may be permissible but don't seal it right up! However from its nature a double hung sash window is hard to seal perfectly (and still work).
    • CommentAuthorbjd
    • CommentTimeSep 24th 2009
     
    Roger - yes it is single glass on the secondary pane.

    Some of the windows have been seriously painted over and are immovable currently (I have freed up a few) - I am wondering if these are actually the ones that mist up the most (can't remember from last winter just now).
    •  
      CommentAuthordjh
    • CommentTimeSep 25th 2009
     
    When my father put the secondary glazing in each winter, he went to a lot of trouble to make sure all the glass was clean and he always did it on a dry day. We didn't have much problem with condensation and he reckoned this was why. The secondary glazing was only over fixed lights.

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